This invention relates to gas monitors.
Investigation of the levels of various pollutants in ambient air requires a reliable means of sampling the air and quantitatively determining the presence or concentration of the pollutant of interest. One method of sampling involves continuous monitoring of flowing gases: for example, in one such monitor, carbon monoxide (CO) concentration in stack gases is determined by passing the stack gases over a bed of I.sub.2 O.sub.5, liberating I.sub.2 into the gas stream which is then passed directly over the platinum cathode of a galvanic cell having a halide electrolyte; the concentration of CO is determined from the current generated by the presence of I.sub.2 in the electrolyte (U.S. Pat. No. 3,909,204). Another monitor, used for determining ozone concentration in flowing gases, involves direction of the gas flow into a galvanic cell having a potassium iodide (KI)-containing electrolyte; the reaction of ozone and KI is according to the following expression: EQU 2KI+O.sub.3 .fwdarw.K.sub.2 O+O.sub.2 +I.sub.2
yielding free iodine (I.sub.2) that can be determined by applying a voltage across the cell and measuring the resulting current (U.S. Pat. No, 3,888,754).
Another sampling method pumps a known volume of air into a detection device capable of measuring the amount of a given analyte gas in that sample. Still another widely-used sampling technique relies upon molecular diffusion of gases: this passive type of sampler has proven to be comparatively simple, lightweight, easy to use, inexpensive and nonhazardous, and therefore more suitable for certain applications. Multiple passive gas monitors can be used to make simultaneous multi-point measurements or repeated measurements over time at a single location, and are therefore an important tool for large-scale air pollution studies involving wide temporal or spatial variables. Also, the passive design may be incorporated into a device intended to monitor an individual's personal exposure to a given pollutant over a period of time.
Generally, the passive gas sampler consists of a collection apparatus containing a collecting medium. The apparatus is typically a container with an orifice at one end to permit ambient gases to diffuse in and thereby contact the collecting medium.
The collecting medium may take various forms. Some trap molecules of pollutant gas by adsorption, later releasing the gas upon heating, reduction of atmospheric pressure, or treatment with a solvent. For example, benzene vapor may be adsorbed to an activated charcoal collecting medium for subsequent release and analysis. In another type of monitor, the collecting medium may be a reagent that traps the pollutant gas by chemically reacting with it; one example is triethanolamine, a liquid reagent used to collect NO.sub.2 (e.g., Palmes et al., Am. Ind. Hyg. Ass. J. 37:570-577, 1976; Yanagisawa et al., Environ. Int. 8:235-242, 1982). Suzuki et al (Taiki Osen Gakkaishi 18:544. 1983) describe a passive monitor for measuring the concentration of oxidant gases that utilizes a filter paper impregnated with 10% neutral-buffered KI aqueous solution. Exposure of the monitor to oxidant gases causes KI on the filter paper to undergo a reaction, liberating I.sub.2 which is extracted from the filter paper and titrated with a sodium thiosulfate solution to determine the amount of I.sub.2 present, a measure of the amount of ozone or other oxidant gas to which the monitor has been exposed. The accuracy of this technique depends in part upon the degree of sublimation of liberated I.sub.2 during exposure, storage and analysis of the monitor. Another type of passive gas monitor is described by U.S Pat. No. 4,267,023, wherein gases diffuse into an electrolyte solution held in a wearable container. A measurable chemical reaction occurs between certain gas species and a constituent of the electrolyte (e.g., SO.sub.2 gas will react with HgBr.sub.2 in the electrolyte to form Br ions, which can be quantified in an electrochemical cell.